Well, it's a bit of a story.
You see, through elementary and middle school, I'd tricked my friends into thinking I was smart. Sure, I got good grades, but I'm actually a grade A doofus. Anyways, one of my friends at the time was really into fanfiction. She wanted to be an author when she grew up, and liked writing stories for the shows she watched (we were 12, give us a break).
Anyways, at some point it popped into her head that if I have an account, I'm more likely to read and comment on her stories (which turned out to be wrong btw), so she went and used my email to make me an account. What was the name of the account? Einstien, as a joke to me being 'smart'
She shows it to me, and I think 'cool I guess?' and continue on with life. A couple weeks later we both realize it's spelled wrong, but the real einstein was taken so we kept it. That's where the 'Einstien' comes from.
But what about the 74???
Well, a year late, being 13 and Minecraft being big as it was (I think it was still in the beta if I remember correctly...), I got minecraft and chose to use einstien as my name, as it had stuck. Except, another idiot who can't spell had already taken it, so I needed a number. One of the best lines in the entire show of Spongebob is when he's telling patrick how many good noodle stars he has (74). The stupid way he says 74 has made it my favorite number so it stuck.
From then on for the next... 5 years? I've used einstien74 as my username.
Then I realized it makes me sound like a prick who thinks he's smart, so I made a new username. It was too late though, I'd already made a smogon account, so I'm using Einstien74 instead of my new preferred Quemarco (which is a username story for another day).
Of course, Nice thing about a name like Einstien74 is it shortens quite nicely to Ein